Providing for Motor Short Circuit Forces in Its Foundation Design

What forces do you think a short circuit will create?

The winding short circuits can give rise to very high currents which unlike normal currents may tend to produce very high internal forces.For large and HV motors these forces could be very large.However, these forces will mainly act on the conductors in the slots and though not produced by the mechanical load may produce a serious torque disturbance due to a sudden voltage dip.Also due to short circuit protections provided for the motors the supply would be put off in a very short time. However I really could not imagine how to calculate the forces that me arise due to this which will put extra burden on the foundations.The question was asked by a civil engineer friend who designs foundations and he wanted some authentic reply which could be supported by some technical papers or calculations etc. If any written material is available on this , I would be very happy. Thanks.

The way i see it, if there is a short (between windings of different phases or from one winding to earth), would not the winding impedance up to that point attenuate the short-circuit current? The current will be maximum when the short is at the incoming terminals, but then the motor is completely out of it.

Anyway, some knowledgeable motor designers may have some better information for you. Calculation of electro-dynamic forces due to short circuits is well covered in many articles on the web, my own favourite is ECT 158 from Schneider Electric, mainly applicable to switchgear.

Foundation design is not normally done by electrical engineers. I recommend posting your question in the Civil Engineering forum.

I do agree with you. But at the same time the information on electrodynamic forces due to a short circuit inside the motor windings has to be provided by an electrical expert in the field.It is also possible that these forces could be quite small compared to other major factors on which the foundation design depends and hence is automatically taken care of in the overall design. It is obviously so, since I do not find this factor mentioned in design of motor foundations.But if somebody really wants to calculate it then I think only the particular motor manufacturer could provide the necessary details.

I thank everybody for showing interest in the subject and incase somebody has any further information on the subject it would be welcome since the basice question still remains unanswered.

You still haven't posted any images.....that might make it clearer

Anyway, a motor is an end-load. Unlike a circuit breaker, which just passes electricity through itself. A short-circuit at the downstream side of a CB will be VERY high since a CB has negligible impedance. A motor on the other hand, is already shorted at the end (if star-connected). It has considerable impedance. So, you have to rethink what i already said in the earlier post....what is the likely magnitude of a short-circuit within a motor? Not much, i think.

Well Said! I do fully agree that when a civil engineer designs a foundation for a motor a lot of cushion & saftey factors are alreday added in the design. When a short circuit happens inside a motor, it is very true that there would be electrdynamic stresses, but a majority of these stresses would be spnet in trying to detach the windings from the core and as such only a little stress might pass on to the foundation thro the motor body, which also would have abosorbed part of the stress.

However, if your civil engineering friend wants to be very precise to calculate such short circuit stresses too, while designing the foundation, there are good references available (as the one quoted by KVS) to calculate the electro-dynamic stresses developed during short circuit and they can be taken as a guide line value.

But, thinking practically, if such a practice of calculating short circuit stresses is necessary for foundation design , is it not also for any other electrical equipment founation like switchboards, transformers, towers, generators and the like, apart from motor foundations? I have rarely come across any such query in the past from any of my civil engineering colleagues while ther were designing foundations for electrical equipments.

You can usually ignore short circuit calculations for motors up to about 200 hp. The electrodynamic forces should be within the capacity of a foundation design based on the mass of the motor. Short circuit stresses must be considered for larger motors. The motor manufacturer should be able to provide the necessary data to allow a competent civil engineer to design a foundation for their motor.

You should also remember that the most common failure is not the foundation or the motor, but the anchor bolts connecting the two. I've seen the results when a motor rolled because someone went cheap on fasteners.